1. Field of the Invention
The invention relates to a method of inspecting laminated core stacks of electric machines for interlamination shorts, in which method the core stack is magnetized by means of an auxiliary winding and the iron surface is scanned by means of a measuring coil arrangement with downstream measuring instrument.
The invention relates furthermore to an apparatus for carrying out the method.
The starting point for the invention is a prior art as it emerges, for example, from the technical automobile journal "ELIN-Zeitschrift" 1984, No. 1/2, page 58, section entitled "Iron short separation of stator core stacks".
2. Discussion of Background
Laminated stator core bodies, in particular of electric machines, are inspected for interlamination shorts during manufacture and in operation in the course of maintenance operations using the measuring method comprising ring excitation of the stator lamination with rated induction. This method, which indicates the effect of currents due to interlamination shorts by local temperature differences, requires a high-power and regulable high-voltage source and excitation windings with large cross sections.
In the case of stator laminations with built-in winding rods, it is possible to detect only the locations of faults (short circuit of several laminations between each other) at the tooth surface and not the interlaminar faults in the slot bottom and at the slot sides with this inspection. This method detects only interlamination shorts having a particular contact resistance and resulting local temperature differences, and therefore does not detect all the interlamination short locations. The increase in temperature alone is insufficient to assess the interlamination short location quantitatively.
The disadvantages of the conventional iron lamination inspection can be avoided with the method disclosed in the journal "ELIN . . . " mentioned in the introduction of measuring current fields due to interlamination shorts with a weak yoke induction. Only a low-voltage supply connection is required to magnetize the core stack. Under these circumstances, virtually all interlamination shorts can be found, even those which are situated at the slot wall or in the slot bottom. It is also possible to determine the condition of the interlaminar insulation over the entire hole area (also slot area). The core stack can be inspected with a built-in rotor. The known method makes possible a qualified analysis of the interlamination short location. The efficiency of any stack repair carried out can easily be inspected immediately. In addition, the method makes it possible to compare the state of the core stacks of a plurality of machines. Changes in the core stack in the course of time and ageing processes can be detected.
A disadvantage in this case is that, in the case of a core stack fault location, the measured value reading read off at that point is proportional to the fault current ("cause") but not to the excess temperature ("effect") which occurs during operation. Since there is in fact a physical relationship, but not one which is simple to assign, between "cause" and "effect", the interpretation of the measurements requires some experience. In this connection, there is no problem in interpreting sharply defined measured value readings since such sharply defined measured valve readings are invariably due to faults; it is more difficult to interpret diffuse measurement readings since in this case these may be either "hot points" or inhomogeneities in the core stack.
In order to be able to assign measurement readings quantitatively to a wide variety of core stack types, a relative calibration is carried out at the start of every measurement. Small sub-areas of the core stack are superficially short circuited ("calibration fault location") with the excitation switched on and the measured value reading then recorded. A fault reading for an actual core stack fault then makes it possible to infer the actual size of the fault on the basis of the prior relative calibration.